In order to mass-produce optical discs such as those of the compact disc family (e.g., CD Audio, CD ROM, Video CD, CD-I, CD-MO, MD, etc.) a reliable mastering process is required. Unfortunately, conventional compact disc mastering techniques involve numerous complicated and time-consuming steps.
FIG. 1 depicts the basic processing steps in creating a compact disc master from which other discs can be mass-produced. The first step involves providing a circular glass substrate 12 approximately nine and one-half inches in diameter. The substrate 12 must be cleaned, dried and then carefully subjected to a visual inspection for imperfections, surface smoothness and the like.
An adhesive layer coating (not shown) is then placed on one surface of the glass 12 according to strict tolerances. A photo-resistive coating 22 is then deposited over the adhesive layer as shown in step 20. This photo-resistive coating 22 must also be carefully inspected to insure that it is evenly and continuously applied across the surface of the substrate 12.
The glass substrate 12 must then be appropriately cured and sampled to insure that its thickness conforms to the required tolerance. The steps of applying the coating to the substrate and sampling the coating require significant time and must be precisely performed.
After curing, in step 30, laser beam recording (LBR) takes place. LBR involves the selective exposure of the photo-resistive coating to the beam 32 of a laser (not shown) in order to form the appropriate pattern of pits and lands that represent the data recorded on an optical disc. The laser is typically a continuous wave laser with exposure of its beam to the photo-resistive coating 22 controlled by an acusto-optic modulator (AOM) (not shown).
The AOM acts as an electronic shutter to the laser beam and, as is well-known by those skilled in the art, is controlled by a string of binary 1's and 0's generated by an encoder (also not shown). The binary 1's and 0's represent the data being recorded and correspond to the pits and lands formed in a recorded optical disc.
After the photo-resistive coating 22 has been selectively exposed to the laser as described above, the photo-resistive coating 22 must next be developed so that the exposed portions can be removed. Developing is accomplished in step 40 by placing the substrate 12 in a caustic sodium hydroxide solution. Again, after developing the glass must be inspected. This time by measuring the diffraction orders of the tracks, among other things.
The next step 50 is a metalization step and involves placing a thin coating of silver or nickel 52 over the entire surface of the substrate 12 such that it follows the pattern of pits and lands of the now developed photo-resistive layer 22. In the case of silver, this is accomplished by well-known evaporation techniques. The evaporation results in the formation of the metal coating 52 typically 120 nanometers thick. At this point, the metalized substrate or the "glass master" is typically "played" in a specially adapted CD player to insure that the pits have been properly formed. It is only at this point, after the time and expense of each of the above steps, that the accuracy of the recording can be determined. Additionally, a visual inspection takes place to further confirm the accuracy of the production.
In steps 60 and 70, a nickel plating 62 is formed via electroplating over the metalized glass such that a metal master 64, with pits and lands opposite to those of the metalized glass plate, is formed when it is removed from the substrate. This metal master is also known as a "father" and is formed by well-known electroplating procedures.
Another inspection is required at this point to insure the father has been properly formed. Because the extremely thin silver or nickel metalization 52 forming the glass master is lost to the father during the nickel plating which forms the father, there can be only one father.
From the metal master or father 64, a so-called "mother" 82 is formed also out of nickel as shown in step 80. The mother 82 is simply the inverse of the metal master or father and is similarly formed by electroplating. Several mothers can be formed from the metal master or father, however, each must be inspected to insure that it has been properly formed.
Up to this point in the process there have been no less than six detailed inspections involving numerous processing steps. Generally, the processing steps up to this point typically require seven to nine hours of intensive labor.
Finally, in step 90 nickel stampers 92 are formed from the mothers 82. The stampers 92 are again simply an inverse of the mother 82 formed by electroplating the same. From the stampers, compact discs can be manufactured by injection molding melted resin, e.g., optical quality polycarbonate, at high pressure into a mold comprising the stamper and allowing it to solidify.
As can be observed from the above-steps, before mothers 82 or stampers 92 are formed, numerous steps must be undertaken. Unfortunately, any small variation in the process conditions, equipment alignment, etc., can result in failure of the entire process, therefore, yields tend to be less than ideal.
One alternative known by those skilled in the art involves reducing the required number of steps to reach the mother and, therefore, the stamper stage. This alternative, however, begins with first placing a coating over the glass substrate and thus involves steps analogous to steps 10 and 20 as described above. Specifically, a glass substrate 12 is first provided, cleaned, dried and inspected thoroughly. Then a coating is placed along one surface of the glass substrate 12 and cured. However, in this case the coating is a non-photo-resistive coating compared to the photo-resistive coating 22 of FIG. 1.
Next, the coating is selectively subjected to a laser beam. Instead of merely exposing the coating as described in step 30 above, the laser beam in this case actually vaporizes the coating. Thus, pits are formed (and lands are left) directly in the non-photo-resistive coating. Accordingly, there is no need for developing and step 40 can be skipped.
Because the pits are actually formed at this stage, i.e., there is a physical or mechanical change of the coating, a read laser immediately down-stream from the recording laser, i.e., the laser vaporizing selective portions of the coating, can be used to verify the accuracy of the vaporization process by reading or "playing" the disc. Thus, if a serious mistake is made at this early stage, the process can immediately be aborted. Additionally, this laser allows feedback to control exposure, power and other beam parameters of the recording (ablating) laser. While this alternative technique eliminates some processing time, i.e., for the developing process, the glass substrate must still be coated and creation of the father or metal master 64 (steps 60 and 70) must still take place before mothers 82 or stampers 92 can be formed.
Finally, another alternative known by those skilled in the art is to apply a conductive coating to a glass substrate and perform direct laser ablation of this conductive coating. See, U.S. Pat. No. 5,494,782 (incorporated herein by reference). While this alternative avoids the need for metalizing with silver, for example, the troublesome and time consuming steps of properly applying and ablating the coating still must be undertaken. Accordingly, it would be advantageous to be able to directly form a stamper without first forming a father or mother, and without the need to ablate a conductive coating.